Caps can generally be of one-piece type in which the cap material itself forms the seal, by virtue of the shape of the cap, when this latter is screwed onto the container neck; or be of two-piece type in which the central panel of the cap is bounded by a restraining ring containing a gasket of soft material which forms the seal when compressed by being screwed against the container mouth.
However, the development of cap forming methods using hot compression, together with the increasing number of materials used, have highlighted the need to provide the cap with independent sealing means, to be applied during the formation of the cap itself.
The nature of the container contents has also highlighted the need to form not only a mechanical seal tight against liquids, but also a seal effective against gases.
For example, certain types of liquid foods must necessarily be protected against oxidation by preventing undesirable oxygen entry into the container after the cap has been screwed down.
Oxygen can enter by permeation in the form of molecules, and is not prevented by the materials usually used to construct caps.
In the case of carbonated drinks it is also important to seek to limit CO2 migration, which can occur both mechanically and by permeation.
It has been sought to provide a seal against gases, the so-called barrier effect, by constructing or installing a discoidal gasket able to provide the desired barrier effect.
However this solution is not generally satisfactory because the materials used to provide the barrier effect do not possess mechanical characteristics suitable for forming a sufficient seal as their hardness exceeds that necessary to ensure a good mechanical seal against gases and liquids.
The object of the present patent is to provide closure caps, preferably formable by known compression moulding processes, and the relative forming method, which are able to provide an hermetic seal at the mouth of containers both against the contained liquid and against gases, and hence prevent the entry of oxygen and the exit of CO2.